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Abstract:

An exemplary embodiment of the invention provides a molten metal
containment structure, including a vessel having an internal volume for
containing molten metal and an open upper end. The structure further
includes a cover for the vessel having an underside facing the internal
volume, the cover being movable between a closed position, covering the
open upper end of the vessel with the underside of the cover, and an open
position in which the cover is remote from the open upper end of the
vessel to allow access to the internal volume from one side of the
structure. The cover is attached to at least one lifting arm (preferably
two or more) and at least on rotation control arm (preferably two or
more). The lifting arm(s) operates to guide the cover from the open
position to the closed position, and vice versa. The rotation control
arm(s) operates to control pivoting of the cover during movement from the
closed position to the open position, and vice versa, to prevent exposure
of the underside of the cover, which in operation is very hot, to a
person positioned adjacent to the structure at the aforesaid one side
thereof. The lifting arm(s) and/or rotation control arm(s) provide a
force that at least partially counterbalances a weight of the cover.

Claims:

1. A molten metal containment structure, comprising: a vessel having an
internal volume for containing molten metal and an open upper end; and a
cover for the vessel having an underside facing said internal volume,
said cover being movable between a closed position, covering said open
upper end of the vessel with the underside of the cover, and an open
position in which said cover is remote from said open upper end of the
vessel to allow access to the internal volume from one side of the
structure; wherein said cover is attached to at least one elongated
lifting arm and at least one elongated rotation control arm, the at least
one lifting arm operating to guide the cover from said closed position to
said open position, and vice versa, and the at least one rotation control
arm operating to control pivoting of the cover during movement from said
open position to said closed position, and vice versa, to prevent
exposure of said underside of the cover to a person positioned adjacent
to the structure at said one side thereof, and wherein said at least one
lifting arm or said at least one rotation control arm provides a force
that at least partially counterbalances weight of the cover.

2. The structure of claim 1, wherein said at least one lifting arm has a
counterweight attached thereto providing said force.

3. The structure of claim 1, wherein said at least one lifting arm has a
spring attached thereto providing said force.

4. The structure of claim 1, wherein said at least one lifting arm is
attached to said vessel via a pivot having a center in a vertical plane,
and wherein said cover and directly attached parts of said arms have a
center of gravity that is positioned on one side of said vertical plane
when in the closed position and the other side of said plane when in the
open position.

5. The structure of claim 1, wherein said at least one lifting arm is
pivotally attached to said vessel at a vertical height below a vertical
height of said open upper end of the vessel.

6. The structure of claim 1, wherein said underside of the cover is
provided with a heater for heating said internal volume when said cover
is in the closed position.

7. The structure of claim 1, wherein said underside of the cover is
provided solely with a layer of refractory insulating material facing
said internal volume.

8. The structure of claim 1, having at least two lifting arms and at
least two rotation control arms, said lifting arms being mutually spaced
apart along the cover, and said rotation control arms being mutually
spaced apart along the cover.

9. The structure of claim 8, wherein each lifting arm is positioned
immediately adjacent to a rotation control arm, and vice versa, whereby
said arms provide mutual physical support during movement of the cover.

10. The structure of claim 1, wherein said at least one lifting arm has a
C-shaped portion to accommodate movement of the cover from the closed
position to the open position.

11. The structure of claim 1, wherein said at least one rotation control
arm has a C-shaped portion to accommodate movement of the cover from the
closed position to the open position.

12. The structure of claim 1, wherein said cover is provided with a
manually graspable region adjacent to said one side.

13. The structure of claim 12, wherein the manually graspable region is a
handle attached to the cover.

14. The structure of claim 1, including a powered device for moving said
cover from said closed position to said open position, and vice versa.

15. The structure of claim 14, wherein said device includes a piston and
cylinder acting between said at least one lifting arm and said casing.

16. The structure of claim 14, wherein said device includes a remotely
operable control, said control being operable to control movement of said
cover.

17. The structure of claim 1, wherein said vessel is made of a refractory
material that is resistant to attack by molten metal.

18. The structure of claim 1, wherein said at least one rotation control
arm prevents said cover from rotating at said underside from a horizontal
position by more than 25.degree. towards said one side.

19. The structure of claim 1, wherein said at least one rotation control
arm causes said cover to rotate at said underside from a horizontal
position in a direction facing away from said one side.

20. The structure of claim 1, wherein said vessel is an elongated trough
for conveying molten metal, said trough having an inlet for molten metal
at one end of the structure and an outlet for molten metal at an opposite
end of the structure.

21. The structure of claim 1, wherein said vessel includes an outer metal
casing.

22. The structure of claim 21, wherein said arms are pivotally attached
to a sidewall of said metal casing opposite to said one side of the
structure.

Description:

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the priority right of prior U.S.
provisional patent application Ser. No. 61/335,986 filed Jan. 13, 2010 by
applicants named herein. The entire contents of U.S. application Ser. No.
61/335,986 are specifically incorporated herein by this reference for all
purposes.

BACKGROUND OF THE INVENTION

[0002] I. Field of the Invention

[0003] This invention relates to molten metal containment structures used
for conveying, treating or holding molten metals, incorporating
refractory or ceramic molten metal containment vessels, such as troughs
or crucibles. More particularly, the invention relates to the provision
of covers for such structures and vessels.

[0004] II. Background Art

[0005] Molten metal containment structures, e.g. metal-conveying troughs
or launders, are often required during metal treatment or casting
operations to convey molten metal from one location, such as a metal
melting furnace, to another location, such as a casting mold or casting
table. In other operations, such vessels are used for metal treatments,
such as metal filtering, metal degassing or metal transportation.

[0006] When the vessels contained in such structures are open at the top,
it is desirable to provide a cover both to keep heat in and to keep
foreign objects and debris out. Sometimes, such a cover is provided with
heaters to heat the vessel in advance of molten metal entry, or to
maintain the high temperature of the molten metal as it is being treated
or conveyed. It is desirable to make such covers removable so that
operators may obtain access to the interior of the vessel, e.g. to clean
out the vessel, to remove blockages or to assess molten metal height or
condition. The covers may be loose, thereby requiring the operator to
lift and move the covers manually, or they may be attached to the
structure by hinges typically on the side of the vessel most distant from
the operator.

[0007] A disadvantage of providing such removable or openable covers is
that their undersides, i.e. the surfaces facing the molten metal, become
very hot in use and operators of the equipment may be exposed to heat
radiating from the undersides when the covers are lifted, resulting in
discomfort or burns. In operation with molten aluminum alloys, for
example, the underside of a cover may have a temperature within the range
of 400 to 800° C. when raised. Moreover, a simple hinge may result
in crowding of the opening by the cover, even when the cover is fully
retracted. Covers of this kind are often made of a metal shell with a
refractory lining and may incorporate heaters of one kind or another. As
a result, such covers can be quite heavy, so the lifting may be difficult
when attempted by hand, and damage may be caused (particularly to
integral heaters) if the covers are simply allowed to fall back into
place.

[0008] An example of a known metal-conveying trough provided with a cover
is disclosed in U.S. Pat. No. 4,531,717 which issued to Georges Hebrant
on Jul. 30, 1985. The elongated trough is provided with a number of
covers attached to the trough by hinges. The covers are provided with
heating means to pre-heat the trough. The force required to open the
covers is somewhat reduced by springs which tend to urge the cover to the
open (raised) position. However, the operator is exposed directly to the
hot underside of the cover when the cover is raised.

[0009] Another example of a known device is disclosed in U.S. patent
publication 2008/0230965 A1, which was published on Sep. 25, 2008 naming
Robert Gorza, et al., as inventors. This publication discloses a
floor-mounted handling device, which is a kind of small crane, for
removing covers from metal conveying troughs and runners. The device can
handle covers of virtually any weight and avoids exposing operators to
hot environments. However, the apparatus is costly and requires clear
floor space around the troughs or runners.

[0010] There is a need for additional approaches for addressing some or
all of the problems mentioned above.

SUMMARY OF THE EXEMPLARY EMBODIMENTS

[0011] An exemplary embodiment of the invention provides a molten metal
containment structure, including a vessel, preferably provided with an
outer metal casing, having an internal volume for containing molten metal
and an open upper end. The structure further includes a cover for the
vessel having an underside facing the internal volume, the cover being
movable between a closed position, covering the open upper end of the
vessel with the underside of the cover, and an open position in which
said cover is remote from the open upper end of the vessel to allow
access to the internal volume from one side of the structure. The cover
is attached to a support by at least one elongated lifting arm
(preferably two or more) and at least one elongated rotation control arm
(preferably two or more). The lifting arm operates to guide the cover
from the closed position to the open position, and vice versa, and the
rotation control arm operates to control horizontal axial pivoting of the
cover during movement from the closed position to the open position, and
vice versa, to prevent exposure of the underside of the cover, which in
operation becomes very hot, to a person positioned adjacent to the
structure at the aforesaid one side thereof. Either the lifting arm(s)
and/or the rotation control arm(s) provide a force that at least
partially counterbalances a weight of the cover. The counterbalancing
weight is most preferably provided by the lifting arm(s).

[0012] The elongated arms are preferably pivotally attached at one end to
the cover and at an opposite end to a solid support, which may be part of
the structure (e.g. a sidewall of the metal casing opposite to the side
at which the operator stands), or another item, e.g. an adjacent wall or
piece of machinery.

[0013] The rotation control arm(s) preferably prevents the cover from
rotating at the underside from a horizontal position by more than
25° towards the one side of the structure where the operator
stands. More preferably, the underside of the cover is kept horizontal or
is rotated in a direction away from the operator side of the structure.

[0014] The vessel is preferably a metal-conveying trough made of a
refractory material suitable for conveying molten metals, the trough
having a channel for molten metal extending from one longitudinal end of
the structure to the other. The cover preferably has a metal shell and an
interior layer of insulating refractory material. However, the vessel may
be any other kind of vessel for holding, treating or conveying molten
metal having an open top that may be closed by a cover.

[0015] The sets of arms are preferably such that:

[0016] 1) The cover is lifted completely off the open end of the vessel;

[0017] 2) The cover is moved away from the open upper end of the vessel to
expose the entire open upper end;

[0018] 3) The hot underside of the cover is maintained in an orientation
that does not expose the operator to its radiant heat; and

[0019] 4) The counterbalancing force provides assistance to the operator
in moving the trough in an opening direction and preferably also a
closing direction.

[0020] Most preferably, the arms and the counterbalancing force are such
that the cover remains securely in both the closed and the open positions
when moved between the two. The counterbalancing force may be provided by
a counterweight or by springs or other means. The arms are preferably
shaped to avoid contact with the cover during movement of the cover from
the closed position to the open position, and vice versa (e.g. they may
be C-shaped adjacent to the cover). Preferably, the lifting arm(s) is
attached to said vessel via a pivot having a center in a vertical plane
such that the cover and parts of the arms directly attached to the cover
have a center of gravity that is positioned on one side of the vertical
plane when in the closed position and the other side of said plane when
in the open position.

[0021] Most preferably, the lifting arm(s) is pivotally attached to the
vessel at a vertical height below the vertical height of the open upper
end of the vessel so that the cover moves in a flat arc between the open
and closed positions.

[0022] The underside of the cover may be provided with a heater for
heating the internal volume when the cover is in the closed position, or
alternatively, there may be no such heater and the underside may be
provided simply with a layer of refractory insulating material facing the
internal volume of the structure.

[0023] When the structure has at least two lifting arms and at least two
rotation control arms, they are preferably the same in number, and each
lifting arm is preferably positioned immediately adjacent to a rotation
control arm, and vice versa, so that the arms provide mutual physical
support in the lateral direction during movement of the cover.

[0024] The cover is preferably provided with a manually graspable region,
such as a handle, adjacent to the one side to facilitate opening and
closing of the cover. Alternatively, a powered device, such as a
hydraulic, pneumatic or electrical motor, may be provided so that the
cover can be moved without direct contact from the human operator. Such a
device would preferably be provided with a control, either wired or
wireless, and such a control may be located on the structure (preferably
at the one side thereof) or elsewhere.

[0025] The vessel of the structure is preferably made of a refractory
material that is resistant to attack by molten metal, e.g. a refractory
ceramic material.

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Exemplary embodiments of the invention are described in more detail
below with reference to the accompanying drawings, in which:

[0027] FIG. 1 is an end view of a molten metal trough structure according
to one exemplary embodiment of the invention provided with a cover in the
fully closed position;

[0028] FIG. 2 is an end view of the structure of FIG. 1, but showing the
cover in a fully open position;

[0029] FIG. 3 is a perspective view of the structure of FIG. 1, i.e. with
the cover closed;

[0030] FIG. 4 is a perspective view of the structure of FIG. 2, i.e. with
the cover open;

[0031] FIG. 5 is a schematic diagram showing movements of the lifting arms
of the embodiment of FIG. 1 from the closed position (the rotation
control arms have been omitted from this drawing for clarity and
intermediate positions are shown in phantom);

[0032]FIG. 6 is a schematic diagram showing the lifting arms of the
embodiment of FIG. 1 in the fully open position (again the rotation
control arms have been omitted and the cover is shown in a horizontal
position);

[0033] FIG. 7 is a schematic diagram showing movements of the rotation
control arms of the embodiment of FIG. 1 from the closed position (the
lifting arms have been largely omitted for clarity);

[0034]FIG. 8 is a schematic diagram showing the rotation-control arms of
the embodiment of FIG. 1 in the fully open position (again the lifting
arms have been largely omitted);

[0035] FIG. 9 is a view similar to FIG. 5 but showing an alternative
exemplary embodiment having a spring rather than a counterweight acting
to counterbalance the weight of the cover (rotation control arms have
again been omitted for clarity);

[0036] FIG. 10 is an enlarged side view of a stable means of attachment of
a lifting arm with the cover (shown partly in cross-section) allowing a
single such arm to be used instead of a pair, this view is from the
operator side of the cover;

[0037] FIG. 11 is a view similar to FIG. 5, but showing a device for
moving the cover from an open position to a closed position, and vice
versa (again rotation control arms have been omitted for clarity); and

[0038] FIG. 12 is an enlarged sectional view of a cover, towards one end
thereof, the cover being suitable for use in the exemplary embodiments
illustrating a heating device provided at the underside of the cover.

DETAILED DESCRIPTION

[0039] FIGS. 1 through 4 of the accompanying drawings illustrate one
exemplary embodiment of the invention. These drawings show a molten metal
(e.g. aluminum) containment structure 10 in the form of a molten-metal
conveying trough structure (metal transfer trough), sometimes referred to
as a launder or metal runner, for conveying molten metal from an entrance
at one end of the structure to an exit at the opposite end. The
illustrated structure 10 includes a metal-conveying trough 12 provided
with channel 14 creating an internal volume for containing and conveying
molten metal from one side of the structure to the other. The trough 12,
which may be composed on one or more trough sections joined end-to-end,
may be made of a suitable molten metal-resistant refractory material,
e.g. oxides of aluminum (e.g. alumina), silica (silica, particularly
fused silica), magnesium (magnesia), calcium (lime), zirconium
(zirconia), boron (boron oxide); metal carbides, borides, nitrides,
silicides, such as silicon carbide, particularly nitride-bonded silicon
carbide (SiC/Si3N4), boron carbide, boron nitride;
aluminosilicates, e.g. calcium aluminum silicate; composite materials
(e.g. composites of oxides and non-oxides); glasses, including machinable
glasses; mineral wools of fibers or mixtures thereof; carbon or graphite;
and the like. In operation, such a structure would be attached at one end
to a source of molten metal, such as a launder from a metal melting
furnace, and at the other end to a remote user of the molten metal, e.g.
a casting table or casting mold. The structure may be made in any
suitable length depending on the distance to be spanned.

[0040] The trough 12 is contained within a metal casing 15 (sometimes
referred to as a "shell", and shown in broken lines in FIGS. 1 and 2) and
there is normally insulating refractory material (not shown) packed
between the metal casing and the trough to reduce heat loss from the
trough and to lower the temperature of the casing. The ends of the
structure are provided with end plates 16 which are larger than the
casing 15. Metal top plates 17 are firmly bolted to the casing 15 by
means of bolts 18. The metal casing 15 may itself be held within
longitudinally-spaced U-shaped axial metal ribs or cradles (not shown)
that raise the casing 15 and the end plates 16 from the underlying floor
56 (see FIG. 6) or other support.

[0041] The channel 14 has an open upper end 19 (FIG. 2) extending along
its length and the open upper end is closed by a movable cover 20 having
an outer metal shell 21 and an internal layer 22 of insulating material.
The cover 20 also has an upstanding external handle 24, forming a
manually-graspable region of the cover, adjacent to one edge of the cover
(the edge closest to a front side 26 of the structure before which the
operator stands when operating the cover), although in other exemplary
embodiments, the cover may be provided with other kinds of graspable
regions for opening the cover, e.g. an overhanging metal lip or channel,
a groove in a side region of the cover, a handle in the side region, an
outwardly extending post, or the like, or may be left without such a
graspable region, particularly if a powered device is provided to open
and close the cover (as described later). The cover has an underside 25
that directly faces the contents of the trough when the cover is in the
closed position, and may therefore be heated to an elevated temperature
by proximity with the molten metal contents of the trough (not shown).
Additionally, the underside 25 may be provided with trough heater(s),
e.g. heaters 66 of the kind shown in FIG. 12 or as described in U.S. Pat.
No. 4,531,717 to Hebrant mentioned above (the disclosure of which patent
is incorporated herein by this reference), and may be elevated in
temperature additionally by the operation of such heater(s).

[0042] The cover 20 is not attached to the top plates 17 of the structure,
but is pivotally attached by a system of four elongated arms to an
upright side wall 27 of the metal casing 15 (see FIGS. 3 and 4), which is
the side wall opposite to the front side 26 of the structure before which
the operator stands during opening or closing of the cover. The four arms
are of two kinds, namely lifting arms 28 and rotation control arms 29.
These allow the cover 20 to be moved from a closed position covering the
channel 14, as shown in FIGS. 1 and 3, to an open position remote from
the channel, as shown in FIGS. 2 and 4. During the movement of the cover
20 from the closed position to the open position, and vice versa, the
underside 25 of the cover is directed away from an operator 55 (see FIGS.
5 through 8) positioned at the front side of the structure. The operator
is therefore not directly exposed to the hot underside 25 when moving the
cover.

[0043] Each of the lifting arms 28 is elongated and is pivotally attached
at one end to the cover 20 and at the other end to a counterweight 30.
Between the two ends, the arms each have an attachment point 32 that
pivotally attaches the arms to a rigid support 37 fixed to the upright
side wall 27 of the metal casing 15. The cover 20 has a bracket 33 (best
seen in FIG. 3) securely attached to the outer metal shell 21. The
bracket 33 has upstanding ends 34 that act as journals for pivotal
attachment of the ends of the arms 28 by means of bolts 35 that extend
through holes in the upstanding ends of the brackets and the adjacent
ends of the arms.

[0044] Each of the rotation control arms 29 is also elongated and has
opposite ends. One end of each arm is pivotally attached to the cover 20
and the other end is pivotally attached to the rigid support 37 via a
pivot point 36 attached to the upright side wall 27 of the metal casing
15. The attachment of arms 29 to the cover 20 is via a bracket 38 firmly
attached to the outer metal shell 21 of the cover. The bracket 38 has
upstanding ends 39 which act as journals for bolts 40 passing through
holes in the upstanding ends 39 and the upper ends of the arms 29,
thereby permitting rotation of the ends of the arms relative to the cover
20. The arms 29 have elbows 42 that are connected together by a tie-rod
43 that provides the arms with additional lateral stability.

[0045] As noted, there are two lifting arms 28 and two rotation-control
arms 29 in the illustrated exemplary embodiment. Each arm of a pair is
separated from the other in the longitudinal direction (direction along
the trough) of the cover, although each of the lifting arms 28 is
positioned directly adjacent to one of the rotation-control arms 29, as
shown. This arrangement reduces end-to-end tilting and swinging of the
cover both because of the longitudinally spaced attachment points of the
arms and the cover, and because of lateral support of the lifting arms by
the rotation-control arms, and vice versa.

[0046] The arms 28 and 29 provide lifting and rotational control of the
cover 20 and constrain the cover to move in a particular path and with a
preferred tilt. The way in which the lifting arms 28 operate in this
regard is illustrated schematically in FIGS. 5 and 6, and the way in
which the rotation-control arms 29 operate is shown in FIGS. 7 and 8. It
should be kept in mind that the arms 28 and 29 are used together in the
illustrated embodiments, although, for reasons of clarity, the lifting
arms 28 are shown in isolation in FIGS. 5 and 6, and the rotation control
arms 29 are shown largely in isolation in FIGS. 7 and 8.

[0047] Referring to FIGS. 5 and 6, it will be seen that each arm 28 has
ends 28A and 28B and intermediate pivot points 32 firmly attached to the
structure 10 (via bracket 37, not shown in these figures). FIG. 5 shows
the cover in the closed position, but illustrates in shadow lines
progressive intermediate positions 28-01 to 28-04 of the lifting arm 28
as an operator lifts the cover using handle 24. The fully open position
of the cover and the arms is shown in FIG. 6. The line 52 in FIGS. 5 and
6 indicates a vertical plane at the top-dead-center of the pivot point
32. While the center of gravity, shown by point 53, of the combination of
the cover 20 and the parts of the lifting arms 28A on the cover side of
the pivot point 32 remains to the right of the line 52 (the side of the
operator 55), the weight of the cover tends to pull the cover to the
closed position. This weight is partially counterbalanced by the combined
weight of the counterweight 30 and the parts 28B of the lifting arms 28
on the counterweight side of the pivot point 32, which act through a
center of gravity shown by point 54. The counterbalancing effect is based
on weight times distance from the pivot point 32, and it is made such
that a relatively small force is required from the operator 55 to move
the cover 20, which otherwise remains in the closed position. The force
required to raise the cover is made much less than the true weight of the
cover because of the effect of the counterweight 30. Until the cover is
opened to such an extent that the center of gravity 53 of the cover 20
lies to the left of the line 52, there is a downward force on the cover
20 urging it to the closed position of FIG. 5. However, when the center
of gravity 53 moves to the left of the line 52, both the weight of the
cover 20 (and upper arm parts 28A) and the weight of the counterweight 30
(and lower arm parts 28B) urge the cover to the fully open position of
FIG. 6 where the cover remains until the operator closes the cover again.
Means are provided to prevent the lifting arms 28 from pivoting beyond
the fully open position of FIG. 6. Such means may involve contact of the
counterweight 30 with the ground 56, as shown, contact of the cover 20
with the arms 28, or any other stop means. The lifting arms 28 are shaped
as shown to provide the desired lever action around the pivot 32 and to
accommodate the movement of the cover. This means that the upper parts
28A are generally C-shaped in order to accommodate the cover, and the
lower parts 28B are straight but arranged at a sharp angle to the upper
parts around the pivot 32.

[0048] In FIGS. 5 and 6, the cover 20 is shown in a horizontal position,
which might be expected if its rotation were not controlled by rotation
control arms 29. This may give the impression that, if no such
rotation-control arms 29 were provided, the underside 25 of the cover
would always face directly down and would therefore not be directed
towards the operator 55. However, this would not be the case because the
operator would exert a lifting force on the cover via the handle 24, and,
if not for the rotation-control arms 29, this would pivot the cover
around the bolt 35 and expose the operator to the hot underside 25.

[0049] FIGS. 7 and 8 show how the rotation-control arms 29 prevent such
exposure. FIG. 7 shows the cover 20 in the closed position, as well as in
intermediate positions 29-01 to 29-04 shown in shadow lines, and FIG. 8
shows the cover 20 in the fully open position. As can be seen, the arms
29 rotate around pivot point 36, in the direction of the arrow A in FIG.
7, from the closed position of FIG. 7 to the open position of FIG. 8.
This rotation constrains the cover 20 to tilt in the direction of arrow B
in FIG. 8 away from the operator 55. The way in which the cover 20 is
constrained to tilt in this manner depends on (1) the relative lengths of
the arms 28 and 29; (2) the distance between the attachment points of the
arms on the cover 20; (3) the horizontal and/or vertical distance between
the pivot points 32 and 36 on the structure 10; and (4) the vertical
distance between the pivotal attachments at the cover 20 (bolts 35 and
40) and the top of the structure 10. If the angle of rotation is to be
adjusted after installation of the cover, it is simplest to substitute
arms 28 or 29 with alternative arms of different lengths than those
originally present.

[0050] It is also to be noted that the pivot points 32 and 36 of the arms
are positioned at a vertical level below that of the top of the structure
10, and hence below the cover 20. This has the effect that the cover 20
is constrained to move in a flat arc from the closed position to the open
position, and vice versa. Thus, rather than having to lift the cover 20
vertically upwards from the closed position, the operator 55 pushes the
cover mostly horizontally away from the structure 10 as shown by the
arrows C in FIGS. 5 and 7. Conversely, the operator pulls the cover
generally horizontally when moving the cover from the open position to
the closed position. A horizontal force originating from the operator has
quite a long moment arm measured down to the pivot point of the lifting
arms 28, thereby requiring a much lower force from the operator directed
in a more favorable direction. Such a push/pull motion can be delivered
from the operator's core muscles of the torso and legs, instead of
requiring a lifting motion that involves upper body and arm strength. The
direction of the required force is thus less taxing on all operators and
makes it easy to operate the cover for those of smaller stature or lesser
arm strength. Such motion is, of course, further facilitated by the
contribution of the counterweight 30.

[0051] As noted earlier, the lifting arms 28 are attached to the cover via
bracket 33 and is the rotation control arms 29 are attached to the cover
via bracket 39. Such brackets allow the arms to be spaced from each other
by desirable distances. The brackets are preferably positioned such that
the center of gravity of the cover is positioned in the longitudinal
direction of the cover between the ends of the pair of arms 28 as well as
between the ends of the pair of arms 29. Moreover, in the transverse
direction of the cover, as shown in FIGS. 7 and 8, the center of gravity
53 of the cover is also preferably positioned between the two brackets 33
and 39 so that the cover is supported stably.

[0052] The amount of tilt of the cover 20 should always be such that the
operator 55 is protected from direct exposure to heat from the underside
25 of the cover. This normally means that the angle of tilt from the
horizontal should preferably be no more than about 25° towards the
operator (because a small tilt to this maximum towards the operator will
not expose the operator to significant heat as the underside 25 still
faces the structure rather than the operator), and is more preferably
held horizontal or ideally tilted away from the operator by any angle,
e.g. up to about 30° as shown in the drawings.

[0053] Although the exemplary embodiments discussed above employ a
counterweight 30 to reduce the force required to move the cover 20, other
means may be used for the same purpose, e.g. springs. A suitable
arrangement employing a spring 45 is shown in FIG. 9, but other designs
would also be possible.

[0054] Moreover, although pairs of each kind of the arms 28 and 29 have
been illustrated, alternative embodiments may have just one arm 28 and
one arm 29. Such an embodiment is less preferred, but may be effective
when the cover 20 is quite small, or when the journals at the pivot
points are strong and prevent significant play. For example, FIG. 10
shows how a single arm 28 may have sidebars 35A and 35B that pivotally
attach to the upstanding ends of the bracket 33, thereby maintaining
spaced journal points while employing just a single lifting arm 28. The
same kind of attachment may be used for a single rotation control arm 29.
On the other hand, if the cover is particularly long, more than two of
each of the spaced arms 28 and 29 may be provided to ensure that the
cover is stably supported. A suitable number of attachment brackets may
be provided for pivotal attachment of all of the arms. Of course, for
really long trough lengths, more than one cover may be provided, each
with a supporting structure as disclosed above.

[0055] It is also to be noted that, while the "over-center" arrangement of
FIGS. 5 and 6 is particularly preferred, other embodiments may be
provided in which the counterweight exactly balances the cover 20 so that
the cover remains in any position when released, i.e. the open position,
the closed position or any position therebetween, or in which the pivot
points generate significant friction so that any downward force on the
cover (when released) fails to exceed the friction and move the cover
from the open position to the closed position, or allows the cover to
fall slowly.

[0056] As briefly alluded to above, a powered device may be provided to
move the cover between the open and closed positions, so that the
operator may avoid direct contact with the cover. Such an arrangement is
shown in FIG. 11 where a pneumatic or hydraulic piston and cylinder
combination 60 acts between the structure 10 and the lifting arms 28 upon
flow of pneumatic or hydraulic fluid through lines 61 from a motor 62
under control of the operator via a pushbutton or lever 63. While such a
device may be made powerful enough not to need assistance from a
counterweight 30 or springs when moving the cover 20, such a
counterweight or springs are still preferably provided so that lower
power equipment may be employed and smoother operation obtained.

[0057] FIG. 12 is a sketch illustrating a cover 20 provided with a trough
heater 65 at the underside. The cover has an outer metal shell 21 and a
layer of insulation 22 to which the heater is attached. The heater
includes numerous elongated electrical heating elements 66 for directing
radiant heat into the channel 14 of the structure. Covers of this kind
benefit particularly from the controlled and counterbalanced systems as
shown because such heaters tend to be rather delicate and can easily be
damaged if the cover is closed too forcefully or quickly. The illustrated
embodiments make it possible to open and close the cover very smoothly
and relatively slowly, and release of the cover does not cause the cover
to fall rapidly onto the structure. The heaters 65 may be operated
electrically, as indicated, or alternatively by combustion of gas or
liquid fuel.

[0058] In the above embodiments, the structure 10 is in the form of an
elongated molten metal trough of the kind used in molten metal
distribution systems used for conveying molten metal from one location
(e.g. a metal melting furnace) to another location (e.g. a casting mold
or casting table). However, according to other exemplary embodiments,
other kinds of molten metal containment vessels may employed, e.g.
structures used as in-line ceramic filters (e.g. troughs containing
ceramic foam filters) for filtering particulates out of a molten metal
stream as it passes through the structure. Examples of such structures
are disclosed in U.S. Pat. No. 5,673,902 which issued to Aubrey et al. on
Oct. 7, 1997, and PCT publication no. WO 2006/110974 A1 published on Oct.
26, 2006. The disclosures of the aforesaid U.S. patent and PCT
publication are specifically incorporated herein by this reference.

[0059] In another exemplary embodiment, the structure may act as a
container in which molten metal is degassed, e.g. as in a so-called
"Alcan compact metal degasser" as disclosed in PCT patent publication WO
95/21273 published on Aug. 10, 1995 (the disclosure of which is
incorporated herein by reference). The degassing operation removes
hydrogen and other impurities from a molten metal stream as it travels
from a furnace to a casting table. Such a structure includes a
trough-like internal volume for molten metal containment into which
rotatable degasser impellers project from above. In the exemplary
embodiments, the impellers may be incorporated into the cover with the
arms arranged to provide clearance as the cover and impellers are moved
between the open and closed positions. Such structures may be used for
batch processing, or may be part of a metal distribution system attached
to metal conveying vessels. The structure may alternatively be designed
as a refractory ceramic crucible for containing large bodies of molten
metal for transport from one location to another. All such alternative
structures may be used with the exemplary embodiments of the invention
provided they benefit from the provision of a movable cover.